Multifunction biomedical electrodes may be used for a variety of purposes. In some instances, multifunction biomedical electrodes may be used to defibrillate patients whose heart is in a state of fibrillation and in imminent danger of death. Multifunction biomedical electrodes may also be used for external pacing of the heart. Multifunction biomedical electrodes may also be required to function as ECG (electrocardiograph) monitoring electrodes when not providing defibrillation or pacing functions.
These requirements make the design of a multifunction biomedical electrode difficult, especially when, for example, the electrodes are required to provide cardiac pacing for extended periods of time (e.g., twelve hours or more). Among the design constraints to be considered are that in, e.g., defibrillation, electrical energies of up to 360 Joules may be delivered repeatedly to a patient using the multifunction biomedical electrode. At such energy levels, currents in the range of 30 to 60 amperes in amplitude and 4–10 milliseconds duration are not uncommon.
Attempts to provide multifunction biomedical electrodes have included an electrically conductive sheet with a lead wire attached. One side of this conductor has an ionically conductive electrolyte usually in the form of a hydrogel. The conductive sheet is typically a metal foil (e.g., tin), in which case exposed strands of one end of the lead wire are anchored to the metal foil to provide a mechanical and electrical connection by means of a rivet, pressure sensitive tape, solder or electrically conductive glue (epoxy, adhesive) etc. The connection between the strands and the metal foil may be made on one of the two sides of the foil or by piercing the foil at any location. Because the metal foil is highly conductive, the location of the connection site on the foil only minimally affects the current distribution from the face of the side of the foil in contact with the electrolyte and then the skin of the patient.
One potential disadvantage of this approach is that the metal foil may reduce the effectiveness of radiological images taken through the multifunction biomedical electrode because of the shadow cast by the metal foil in any such radiological images.
To address radiological imaging issues, one approach has involved replacement of the metal foil by an electrically conductive polymeric sheet as discussed in, e.g., U.S. Pat. No. 5,571,165 to Ferrari. Multifunction biomedical electrodes with conductive polymeric sheets such as those described in U.S. Pat. No. 5,571,165 to Ferrari purportedly address issues related to irritation and burning of patients' skin around the perimeter of the electrode when used in defibrillation. However, such electrodes subjected to the passage of multiple defibrillation pulses (of, e.g., 100 to 360 Joules) exhibit hot spots of temperature rise directly under the stranded lead wire connection in the central part of the electrode.